Also touching the throttle temporarily, it remembers the speed you set it at, and allows the car to return to this speed after you have released the throttle pedal. This is intended for overtaking. Also touching the brake pedal cancels it.
Tip is it takes a second or two to kick in. I find best to hold the throttle pedal where it is, touch the 'on' button, wait a couple of secs, then release, otherwise you get an awkward surging sensation as the comp reads the speed, you slow by say 3 mph, it actuates the mechanism to hold revs appropriate for the original set speed, realises you're doing 3mph too slow, then gives a little stab of throttle to bring you back up to the original speed. Slight 'bunny-hop' sensation which can feel like you're on a driving lesson.
Also you can use the 'go' button as a throttle pedal. Just hold it in (over 18-ish mph) and the car will accelerate, in an Auto this is particularly delightful experience as you've no gear changing to worry about. I find the way it speeds up is neither too slow to be glacial/boring, nor make the instantaneous fuel consumption drop to 7mpg, as it does when I use the loud pedal!!!